1. To make a final decision after a period of consideration. Also seen as "make (one's) mind up." I've made up my mind, and I think we should move after all. I'm not sure what flavor I want—I'm still making my mind up.

2. To become convinced of a particular course of action. Also seen as "make (one's) mind up." Right then and there, I made up my mind that I would become a police officer when I grew up.